The invention relates to an apparatus for injecting additives into a product flow formed by a highly viscous liquid. The invention also relates to a plant with an apparatus of this kind as well as to a method or process which can be performed using the plant.
The highly viscous liquid envisaged is in particular a polymer melt; the additives are low viscosity liquids, in particular lubricants (namely stearates, alcohols, paraffin oils, softeners) which are to be worked into the polymer melts. The difference in viscosities between the melts and the additives is as a rule very large; the viscosities can differ by a factor on the order of magnitude of two or more powers of ten.
When mixing lubricants into a polymer melt, the former must eventually enter homogeneously distributed into solution. As a rule a proportion of about 0.1 to 6% of dissolved lubricants is provided. A lubricant acts as a partition agent, for example during the injection molding of polymers, which develops its action when the products are being demolded, i.e. removed from the injection tool or mold. The admixing is preferably performed by means of static mixers. A plant is known from JP-A 07 100 825 (see PATENT ABSTRACTS OF JAPAN vol. 950 no. 4) in which a thermoplastic polymer and a liquid additive are mixed by means of static mixers. During this, a bypass flow is formed; the additive is introduced into it; and it is mixed in with an additional static mixer. This first mixture is fed into the main flow upstream of a main mixer so that a homogeneous mixture arises subsequently in the main mixture.
If only one additive must be mixed into a highly viscous liquid flow and it suffices to use only one supply member, the opening of the supply member can be arranged in such a manner that additive enters in the axial direction and centrally into the liquid flow. The admixing takes place without problems. If, on the other hand, the infeed is performed radially, the additive, which is present as a separate liquid phase, is transported to the tube wall--as has been shown in experiments--apparently as a result of hydrodynamic forces. A deposit can form at the tube wall through coalescence of the additive or other unfavorable effects can occur which impair or render the desired mixing impossible.
In many cases the use of a plurality of injection members is required, in particular when different additives must be mixed into the polymer or when supply members are provided as reserves in order to be able to maintain operation when individual supply members fail through replacement by reserve members. In these cases, as a rule, radial supply of the additives is required, i.e. supply members with one supply channel each standing transversely to the direction of flow. This results in the above named problems. It is thus the object of the invention to provide a supply device with radial infeeds for additives in which the formation of deposits and other unfavorable effects do not occur.